The invention relates to a method of mooring a vessel to a subsea-anchoring element, which is connected to the seabed, comprising the step of connecting the anchoring element to the seabed, the anchoring element comprising a chain section which is not longer than half the water depth, the chain section being at or near its free end provided with a coupling chain member. The invention also relates to a subsea anchor line exchange and/or repair method and to a subsea connector for use in said method.
From WO-98/57842 it is known to moor a vessel to the seabed via a suction anchor at the end of which a relatively short chain section is connected. The chain section, which can withstand the abrasive forces acting at the mudline of the anchor upon movements of said chain section, is at its free end connected to a sheave through which a looped multi-strand anchor line, such as a polyester or steel wire cable, runs. The looped anchor line is near the installation vessel connected to a triangular plate, one side of which is attached to a winch. The looped anchor line can easily be replaced by attaching a new anchor line to the old line and taking in the looped anchor line from one side. The known method is especially suitable for repair of anchor lines used in deep water, such as water over 300 meters in depth or more. Although the known system is useful for exchange of the anchor lines without diver intervention, the looped mooring lines will already be put in place upon mooring of the vessel. If the multi-strand anchor lines remain attached in an untentioned state for a longer period, which may pass between installation of the anchor and mooring of the vessel to said anchor line, the wave and current-induced motions in the slack anchor lines may result in damage to the multi-strand lines.
Also during lowering of the suction anchor and the multi-strand line, a risk of twisting of the deployment line and the looped end of the mooring leg may occur, especially in deep water systems.
From IIR ""s conference on moorings and anchors, Aberdeen October 1998, a suction caisson is known which is lowered to the seabed and installed therein, the mooring line being attached to a central deployment pad eye. When the suction caisson has been installed, the subsea connector (female part) at the end of the mooring line is detached via a remote operated vehicle (ROV) and is transferred to the mooring pad eye (male part)which is offset from the centreline of the caisson and which is attached at the end of a short stiff cable that is connected to a side of the suction caisson. As the mooring line is used for deployment of the caisson, it is not possible to pre-install the caisson and to moor the vessel at a later stage. The female part, which is very heavy, can not be tilted by the ROV so it can only receive the male part when it is in a vertical position. The function of the short stiff cable is basically to keep the mooring pad eye (male part) in an upright position. If the cable is to long, the male part would bend over and no connection with the female part is possible.
The known construction is only applicable for temporary mooring of semi-submesibles and is not intended for longterm use, due to the fact that the cable, needed to uphold the male part, is very sensitive to abrasion as it lies completely in the mudline.
Also disconnection/change out of the mooring line is not possible as after installation the connection itself will be lying in the mud, not reachable by the ROV.
From U.S. Pat. No. 4,067,282 an anchoring system is known having a quick disconnect chain element in the form of an explosive link, that can be released during high seas. The anchor chains are released near their mid-parts thereof and can be reconnected above water level.
From FR-1 600 191 it is known to lower a female connector along a pilot line to a pre-installed anchor on the seabed having a rope anchor line part of short length which terminates in a male connector plug. The female connector is coupled to a multi-strand anchor line. The known connector is activated by springs that lock the female part around the male part. With the known system it is not possible to change out or repair an existing damaged or worn multi-strand anchor line by attaching a new multi-strand line to the male connector of the relatively short anchor line part.
It is an object of the present invention to provide a versatile mooring method and anchor line repair method for use in deep water in which a multi-strand mooring line can be attached to an anchoring point at a desired moment in time after installation of the anchor.
It is a further object to provide a mooring system in which the anchor point can be installed in a relatively easy manner.
It is a further object of the present invention to provide a mooring line repair method for deep water, such as water depths of 200 meters or more, by which anchor lines can be easily exchanged and/or repaired.
Thereto the method according to the present invention is characterised by the steps of:
lowering a multi-strand anchor line from a vessel, the multi-strand anchor line being at its fee end attached to a connector, the connector comprising an eye that is attached to the multi-strand line, a connector body comprising two spaced apart wall parts that define a recess there between for accommodating the coupling chain member, and a closing member being movable with respect to the connector body, the closing member being slidable to bridge the space between the wall parts.
attaching the connector to the coupling chain member by placing the chain member between the wall parts and operating the closing member to bridge the space between the wall parts.
By first installing the anchor point and subsequently attaching a multi-strand line, the anchor can be lowered without a need to overboard and control the deployment of the chain-wire connection. During the lowering of the anchor there is no risk that twisting of the deployment wire and the mooring leg will occur as only the deployment wire is used on installation of the anchor. This is of particular significance in deep water systems wherein taut mooring system installation normally requires the use of two vessels, one to lower the anchor and the second to establish a large offset while deploying the mooring leg. The present method reduces the complexity of the installation.
By pre-installing of the anchor points, a large degree of flexibility in vessel scheduling is obtained and fabrication schedules for the anchor points and the mooring leg components may be carried out consecutively.
Pre-installing of the anchor points reduces the need to ensure that the multi-strand lines attached to the anchor points while no mooring occurs, come into contact with the seabed and maintain a minimum tension until hook-up. Because the special connector used according to the invention, the multi-strand line can be attached to a chain-link or a chain-like connecting element of the chain section attached to the anchor. Hereby it is possible to attach a new multi-stand line upon installation but also to exchange existing multi-strand lines that have been damaged or which are at the end of their technical service life. During such an exchange, the old multi-strand line may be cut off or may remain attached to the chain section, the new multi-strand line being coupled to the chain connecting member or to a chain link of the chain section. Because of the chain-compatible construction and simple operation of the connector employed in the present invention, the majority of existing deep water anchoring system which employ a relatively short chain section connected to the anchor and a multi-strand anchor line connected to the chain section, can be installed and or replaced. It is possible to attach a guide wire to the chain section, for instance by a ROV or via the first multi-strand line, a cutting element being lowered along the guide wire to cut the chain downstream from the first multi-strand anchor line. Thereafter the subsea connector with the second multi-strand line for replacement of the first line can be lowered. This can be done along the guide wire. The second multi-strand line is attached via the subsea connector, for instance via intervention of a ROV. It is also possible to guide the new multi-strand anchor line towards the chain section via the first multi-strand line, that needs to be replaced, such that the connector of the second line is guided towards the chain section in an easy manner.
A suitable connector for use in the method of the present invention comprises a shackle for attaching to a multi-strand line and a connector body comprising two spaced apart wall parts defining a recess therebetween for accommodating a chain link, and a closing member being movable with respect to the wall parts, the closing member being rotatable or slidable to bridge the space between the wall parts. The closing member may be hydraulically actuated from the vessel, or hydraulically or mechanically from a ROV.